Conventionally, display devices capable of displaying stereoscopic (3-dimensional) images are proposed. In the stereoscopic image display devices, optical elements such as lenticular lenses or parallax barriers are used to provide different video image information items in space. The lenticular lens is a lens having semispherical-shaped (semicircular-shaped) cylindrical lenses linearly arranged in a certain direction and has a one-dimensional optical action. In order to provide parallax information for an observer by use of the optical action of the lenticular lens, it is preferable to arrange the array direction of the cylindrical lenses in the horizontal direction (a direction in which the left and right eyes are arranged) for the observer. However, in practice, since moiré occurs with respect to wires or the like if a normal display panel is used, the array direction of the cylindrical lenses may be shifted from the horizontal direction and arranged in an inclined state in some cases. This is true in the case of the parallax barrier.
Further, requests for switching a 2-dimensional (2D) image and a 3-dimensional (3D) image by means of the same display device exist and the technique for satisfying the above request is proposed. For example, in a reference (JP-A 2010-224191(KOKAI)), the technique using a birefringence lens utilizing a liquid crystal or the like and a polarization switching cell is described. 2D display and 3D display are switched by electrically switching the polarization state of light emitted from the polarization switching cell. Further, partial 2D/3D display is realized in which a part of 2D display is converted to 3D display by partially switching the polarization state of the polarization switching cell in the display screen.
Further, in the reference, the array direction of the lenses can be set without particularly paying attention to the influence given to the partial 2D/3D display. The boundary between the 2D display region and the 3D display region in the partial 2D/3D display is determined according to the electrode structure of the polarization switching cell. On the other hand, the array direction of the lenses is determined according to the structure of the birefringence lens. Since there is no correlation between both of the structures, they can be independently and freely designed.
Further, in the reference, the technique for switching 2D display and 3D display by use of a liquid crystal lens array element as the birefringence lens is described. The liquid crystal lens array element has rod-like electrodes repetitively arranged on one of the substrates. Then, an electric field distribution is created between the electrodes and an electrode formed on the other substrate opposed thereto. The alignment of a liquid crystal layer is changed according to the electric field distribution and a refractive index distribution acting as a lens is created. Since the lens action can be turned on/off by controlling voltages applied to the electrodes, 2D display and 3D display can be switched. A system in which the alignment direction of liquid crystal molecules is thus controlled according to the electric field is called a liquid crystal gradient index (GRIN) lens system.
However, since the polarization switching cell is required in addition to the lenses in the partial 2D/3D display switching technique using the birefringence lens and polarization switching cell described in the reference, the structure becomes complicated and a cost increase becomes a problem. Therefore, it is desirable to partially realize 2D/3D display switching without using the polarization switching cell.
Further, according to the study by the inventors, the shape of the boundary between the 2D display region and the 3D display region receives a restriction of the array direction (or extending direction) of the lenses when an attempt is made to partially realize 2D/3D display switching by use of a liquid crystal GRIN lens. Particularly, when the lens is inclined in an oblique direction and arranged as described in a reference (Jpn. PCT National Publication No. 2001-501073), a problem occurs in which the boundary between the 2D display region and the 3D display region becomes oblique with respect to the lens extending direction, thus the lens is not suitable for partial 2D/3D display.